1. Field of the Invention
The present invention relates to formation of a bipolar junction transistor with polycrystalline silicon emitter as an access device for a phase change memory cell.
2. Description of Background
There are two major groups in computer memory: non-volatile memory and volatile memory. Constant (or nearly constant) input of energy in order to retain information is not necessary in non-volatile memory but is required in the volatile memory. Thus, non-volatile memory devices contain memory in which the state of the memory elements can be retained for days to decades without power consumption. Examples of non-volatile memory devices are Read Only Memory (ROM), Flash Electrical Erasable Read Only Memory, Ferroelectric Random Access Memory, Magnetic Random Access Memory (MRAM), and Phase Change Memory (PCM). Examples of volatile memory devices include Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).
The present invention is directed to resistive non-volatile memories, such as phase change memory. In phase change memory, information is stored in materials that can be manipulated into different phases. Each of these phases exhibit different electrical properties which can be used for storing information. The amorphous and crystalline phases are typically two phases used for bit storage (1's and 0's) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase.
Chalcogenides are a group of materials commonly utilized as phase change material. This group of materials typically contain a chalcogen (Periodic Table Group 16/VIA) and a more electropositive element. Selenium (Se) and tellurium (Te) are the two most common semiconductors in the group used to produce a chalcogenide when creating a phase change memory cell. An example of this would be Ge2Sb2Te5 (GST), SbTe, and In2Se3. However, some phase change materials do not utilize chalcogen, such as GeSb. Thus, a variety of materials can be used in a phase change material cell as long as they can retain separate amorphous and crystalline states.
Phase change memory arrays are usually configured in a cross-point architecture with the memory element controlled by an access device such as a CMOS transistor or a diode at every junction. One problem in PCM and CMOS integration is the drive current required to create a phase change in the phase change material. Small MOSFET devices necessary to provide high density of bits per unit area do not provide enough current to switch the resistive state of phase change materials. A closely packed diode array can provide enough drive current for a resistive phase change in a memory cell but a significant amount of current crosses over to adjacent memory cells (“cross-talk”) in high-density storage devices. Another possibility is the use of a bipolar junction transistor (BJT). However, the integration of CMOS and BJT arrays has proven difficult. Thus, it is desirable to find a method and structure capable of providing the drive current necessary for resistive phase change in a memory cell, minimizing cross-talk between memory cells, and easily integrated into a memory circuit.